DNA repair synthesis was compared in human diploid fibroblasts (WI-38) for the first 76 hrs after damage with either 45 erg/mm2 irradiation (UV, 254nm) or 10 to the minus 5th power M NA-AAF. The specific activity of DNA was determined in confluent WI-38 cells given hydroxyurea to inhibit DNA replication, UV or NA-AAF to induce repair, and a 4 hr pulse with H3-TdR. The above doses of UV and NA-AAF were chosen to give equal DNA repair synthesis 0-4 hr after damage. Repair synthesis 24-28 hr after UV damage was 20% of the 0-4 hr control, while NA-AAF induced repair was 80% and 60% of the 4 hr value at 24-28 hr and 72-76 hr respectively. When NA-AAF was removed at 4 hr by a medium change, the 24-28 hr and 72-76 hr values for repair synthesis were 50% and 20% of the 4 hr controls, respectively. Up to 4 hr after damage the UV-treated cells showed an almost linear increase in repair synthesis, while the NA-AAF-treated cells showed a lag from 0-1/2 hr, followed by linear incorporation from 1/2-4 hr with a slope steeper than that for UV. Thus, in WI-38 cells, DNA repair synthesis after NA-AAF damage starts more slowly and continues for a longer time than after UV damage.